1. Field of the Invention
The present invention relates to a surface-profile measuring instrument.
More specifically, it relates to a surface-profile measuring instrument that scans a surface of a workpiece to measure a contour, surface roughness, waviness and the like of the workpiece.
2. Description of Related Art
A surface-profile measuring instrument that scans a surface of a workpiece to obtain a three-dimensional shape of the workpiece has been known.
FIG. 19 shows an arrangement of a measuring system 100 (surface-profile measuring instrument) using a scanning probe 130.
The measuring system 100 includes a coordinate measuring machine 110 that moves scanning probe 130, a manually-operated console 150, a motion controller 160 for controlling the movement of the coordinate measuring machine 110 and a host computer 200 that drives the coordinate measuring machine 110 via the motion controller 160 and processes the measurement data obtained by the coordinate measuring machine 110 to acquire the dimension and shape of a workpiece W.
The coordinate measuring machine 110 includes a base 111, a drive mechanism 120 vertically mounted on the base 111 to move the scanning probe 130 three-dimensionally, and a drive sensor (not shown) for detecting the drive amount of the drive mechanism 120.
The drive mechanism 120 includes: two beam supporters 121 provided on both sides of the base 111, the beam supporters having a height in Zm-axis direction (approximately vertical direction) and being slidable in Ym-axis direction (along the sides of the base 111); a beam 122 supported on the upper ends of the beam supporters 121 and having a length in Xm-axis direction; a column 123 provided on the beam 122 in a manner slidable in Xm-axis direction, the column 123 having a guide in Zm-axis direction; and a spindle 124 provided on the column 123 to be slidable therewithin in Zm-axis direction and holding the scanning probe 130 on the lower end thereof.
The drive sensor includes a Ym-axis sensor for detecting the movement of the beam supporter 121 in Ym-axis direction, an Xm-axis sensor for detecting the movement of the column 121 in Xm-axis direction, and a Zm-axis sensor for detecting the movement of the spindle 124 in Zm-axis direction.
As shown in FIG. 20, the scanning probe 130 includes a stylus 131 with a probe (measurement portion) 132 at the tip end thereof and a support 133 that supports the base end of the stylus 131 in a manner slidable in Xp direction, Yp direction and Zp direction.
The support 133 includes a slide mechanism (not shown) having an xp slider, yp slider and zp slider (not shown) and a probe sensor (not shown) for detecting the displacement of the slide mechanism in the respective axis directions and outputting the detected displacement.
The stylus 131 is supported by the slide mechanism in a manner slidable within a predetermined range relative to the support 133.
Such arrangement of the scanning probe 130 is disclosed in, for instance, document 1 (JP 05-256640 A).
With this arrangement, while the probe 132 is in contact with the workpiece surface S by a reference pressing amount Δr, the scanning probe 130 is scan-moved along the workpiece surface S.
Then, a movement locus of the scanning probe 130 can be obtained from the drive amount of the drive mechanism 120.
The movement locus of the scanning probe 130 represents the movement locus of the probe 132. The contact point of the workpiece surface S and the probe 132 is present at a position offset from the movement locus of the center of the probe 132 by a predetermined amount (ΔQ).
The position of the scanning probe 130 detected by the drive sensor and the displacement of the stylus 131 detected by the probe sensor are summed to obtain the position of the probe 132, and the position of the probe 132 is corrected by the predetermined offset value (ΔQ) to calculate the position of the workpiece surface S.
When the workpiece surface is scanned by the surface-profile measuring instrument, inertia is applied on a part of the instrument driven with acceleration.
For instance, when the workpiece W is a circle or an arc, centrifugal force is generated by a circular movement, which deforms the drive mechanism 120 (spindle 124) as shown in FIG. 21.
When the deformation on account of the acceleration occurs, the detection value of the sensor contains error corresponding to the deformation.
For instance, when the centrifugal force is generated, the detection value of the sensor represents inner side of the circle on account of the outward deformation of the spindle 124, which results in radial deviation shown in FIG. 22.
In FIG. 22, L1 is a diameter of a ring gauge and L2 is a measurement data
Such problems become prominent when a high-speed scanning measurement is conducted by a large-size coordinate measuring instrument for measuring, for instance, a vehicle body.
In view of this, following arrangement is disclosed in Document 2 (JP 07-324928 A) for correcting the measurement error generated by acceleration.
In the Document 2, a correction value representing deflection characteristics is calculated in advance as a function of a position of a measurement slider and acceleration of the measurement slider.
For instance, a diameter-known ring gauge is measured at various positions within a measurement area with various accelerations to obtain the function of the acceleration and the deflection characteristics.
When a workpiece is measured, in addition to obtaining detection data by various sensors, the correction value is identified based on the acceleration at the time of the measurement and the detection data is corrected by the correction value.
The measurement error on account of acceleration is thus corrected to obtain an accurate measurement value.
In order to obtain the acceleration at the time of measurement, the Document 2 teaches: a method in which the measured value of the position of the measurement slider is differentiated twice (paragraph 0037, claim 12); and a method in which the acceleration of the measurement slider is detected by providing an acceleration sensor (paragraph 0047, claim 13).
According to the method of the Document 2, the position of the measurement slider is differentiated twice to identify the acceleration during the measurement However, the resolution of the acceleration deteriorates in reverse proportion to square of sampling frequency when the position detection value is differentiated twice.
For instance, if the sampling frequency for detecting the position is decupled, the resolution of the calculated acceleration is degraded to one hundredth of the original, which results in deterioration of the resolution of the correction amount by one hundredth of the original.
Accordingly, the acceleration by double differentiation of the position is not practical and cannot answer to the demand for high-speed and high-accuracy movement
Further, though the acceleration of the measurement slider is obtained by differentiating the position of the measurement slider twice, the acceleration and deformation are generated on a tip end of the spindle 124 or a probe unit during the actual measurement, so that there is naturally a limit on the correction accuracy when focusing on the acceleration of the measurement slider.
Incidentally, though the Document 2 mentions to the provision of the acceleration sensor to obtain the acceleration of the measurement slider, no clear recitation for the performance and providing method are disclosed therein and the provision of acceleration sensor is practically difficult.
For instance, when a circle of 100 mm diameter is scan-measured at scanning speed of 10 mm/sec, centripetal acceleration of approximately 50 μG is generated. However, it is difficult to provide an acceleration sensor capable of detecting 50 μG acceleration for each movement axis (total three acceleration sensors) and, even more, it is impossible to provide acceleration sensors adjacent to the probe unit
As discussed above, since the deformation generated during the scanning measurement cannot be accurately calculated, the deformation cannot be corrected.
Accordingly, the shape of a workpiece cannot be accurately measured during high-speed measurement. In order to accurately measure a workpiece, the scanning speed is restricted to a level without causing deformation.
Especially, since a large-size coordinate measuring instrument 110 capable of high-speed measurement of a large workpiece (such as a vehicle) is in demand, a solution for the above problem has been eagerly desired.